Ligament ruptures, such as to the ACL, are common injuries of the knee. While surgical treatment would ideally prevent degenerative changes and functional disability, joint degeneration has been reported in up to 38% of patients after ACL reconstruction. The unpredictable outcome of operative repair reflects a deficiency in the current knowledge of in-vivo biomechanics of the reconstructed ligaments. This research project is designed to measure in-vivo ACL tension during weight-bearing flexion using a non-invasive imaging technique. Newly developed dual-orthogonal fluoroscopic imaging and MR image- based 3D computer modeling techniques will be used to quantitatively determine in-vivo elongation of the ACL under various weight-bearing conditions during knee flexion. The determined in-vivo ACL elongation will be combined with force-elongation curves of the ACL to determine in-vivo ACL tension under specific weight- bearing conditions. A rigorous validation will also be performed in order to ensure that the ACL force- elongation estimates are accurate. This investigation presents an innovative methodology for measuring in- vivo tension of the knee ligaments. The data on in-vivo ACL tension will provide fundamental knowledge for understanding the biomechanical role of the ACL. The data can be used as quantitative guidelines for developing optimzed surgical ACL reconstruction techniques that reproduce in-vivo biomechanics and, hence, restore normal knee function and prevent degenerative arthritis. The methodology established in this project will include an accurate measurement of in-vivo knee kinematics and an indirect measurement of in-vivo ACL tension. These advanced imaging and 3D modeling techniques will provide invaluable insight towards the investigation of in-vivo functional roles of all knee ligaments and the development of appropriate surgical treatments for these soft tissue injuries. This methodology can be readily extended to investigate and optimize the treatment of soft tissue injures of other articular joints such as the spine, shoulder, wrist and ankle. [unreadable] [unreadable] [unreadable]